In typical inductively coupled plasma ("ICP") systems a strong radio frequency field is generated by an induction coil and energizes a gas as a plasma discharge in a torch device. Such plasma systems are typically used for spectroscopy, treatment of fine powders, melting of materials, chemical reactions and the like. These applications derive from the high temperatures inherently associated with a plasma, e.g., on the order of about 9000 degrees Centigrade.
The gases necessary to sustain an ICP discharge are commonly introduced into a torch constructed of a quartz tube which partially contains the high temperature plasma. In such a torch the tube surrounds the discharge to shape the plasma which is maintained by the radio frequency field created by the induction coil encircling the quartz tube.
U.S. Pat. Nos. Re. 29,304 and 4,266,113 illustrate typical ICP torches that may be used for spectroscopy, comprising three concentric tubes. The plasma-forming gas is passed through the annular space between the innermost tube and the middle tube. The innermost tube, or pipe, terminates near the plasma region and is used for a carrier gas containing the sample substance being injected into the plasma. A cooling gas for the tube assembly, which may be the same type or a different gas than for the plasma, flows between the outermost tube and the middle tube. The induction coil is typically formed of copper tubing and is generally water cooled.
As indicated in the above-identified patents the torch assembly is fixed with respect to the induction coil so that the sample substance is injected axially near the rear end of the coil; i.e., the lower end of the coil in a vertical configuration with the plasma issuing upwards. U.S. Pat. No. 4,578,560 discloses the use of flanges on the bottom ends of the tubes which connect to corresponding flanges of lower mounts. Spacers are placed between the connecting flanges to provide adjustment of the tubes during assembly, fixing the positions for operation.
Generally the plasma discharge must be initiated by a starter device. U.S. Pat. No. 3,324,334 mentions a high energy spark source (at column 5, line 46) but provides no details. In U.S. Pat. No. 3,296,410 a tap from the radio frequency generator is disclosed (FIG. 2 of the referenced patent), but in practice this has not been very reliable for starting. U.S. Pat. No. 4,482,246 teaches the use of a Tesla coil which is relatively expensive. A lower cost device is disclosed in aforementioned U.S. Pat. No. Re. 29,304 whereby a carbon rod is introduced into the open end of the torch where it is heated by the radio frequency field, in turn heating the gas to initiate the plasma (column 5, lines 15-20); however, this device also has proven to be unreliable.
Another problem associated with ICP systems is tuning the radio frequency. A typical circuit is shown in the U.S. Pat. No. Re. 29,304 (FIG. 2). The main oscillator is a "tank" circuit, i.e., an LC circuit, in combination with a vacuum triode tube having a DC power supply on the plate. A second LC circuit includes the induction coil for the ICP, that coil also providing at least part of the inductance for the second LC circuit. Coupling between the circuits is either inductive or capacitive. The two circuits are tuned to similar frequencies to obtain transfer of power.
As indicated in aforementioned U.S. Pat. No. 3,296,410 there is a certain amount of coupling between the plasma and the associated induction coil, the coupling resulting in changes in the frequency (column 4, lines 17-26). The changes may occur as the plasma gases change, for example when the sample substance is injected into the plasma. The result is inefficient transfer of radio frequency power from the main oscillator to the ICP. The '410 patent attempts to solve this by a further inductance in the second circuit, but such an approach clearly does not resolve the problem and either a compromise frequency is chosen or retuning is required during operation.
U.S. Pat. No. 4,629,940 shows the utilization of variable capacitance for retuning in which the retuning is done automatically through feedback circuitry. Although such a system has been quite successful, it generally is cumbersome, expensive, and prone to malfunction.
The ICP is to be distinguished from a different type of radio frequency plasma generator as disclosed, for example, in U.S. Pat. No. 3,648,015, in which the plasma is generated capacitively. A metallic nozzle assembly is attached to the output coil and the plasma is generated from the tip of the nozzle. The plasma-forming gas is provided to the nozzle through its connection to the coil which is formed of piping. The gas and a powder are introduced into the coil pipe at another connection point.
In view of the foregoing a primary object of the present invention is to provide an improved induction plasma generating system that remains stable over a range of operating conditions.
Another object is to provide a novel induction plasma generating system having a precisely regulated power output.
A further object is to provide an improved induction plasma generating system having a constant power output over a range of operating conditions.
Yet another object is to provide an improved power regulation system capable of fast response in maintaining constant voltage as load conditions vary.